Date: Sept 14
Sections 6.1, 6.3, 6.5
Conservation of Mass and
Balancing Equations
Conservation of Mass
The Law of Conservation of Mass states that in a chemical reaction, the total mass of the reactants is always equal to the total mass of the products.
In the reaction of zinc and hydrochloric acid, if one doesn't take steps to ensure that the hydrogen gas
does not escape it would seem that there was a mass loss. Because the hydrogen gas does escape then the mass of the products will not be equal to the mass of the products. Whenever a
mesurement is made to verify the Law, the system must be closed. The reaction is done in a sealed container.
In this reaction trapping the hydrogen gas would be dangerous and not advised.
Word Equations to Skeleton Equations
Word equations represent a chemical reaction. It tells you what the reactants are and what products are produced. The left side is the reactants and
the right is the products. When coverting the word equation to the skeleton equation you simpily write the chemical formula for each work, making sure you follow the valence rules and criss-cross the oxidation states propely. When finished you have the skeletal equation.
Using the above mentioned reaction:
zinc + hydrochloric acid ------> hydrogen gas + zinc chloride, becomes
Zn + HCl ------> H2 + ZnCl2
Here I have written a word equation of what takes place and then wrote the formula of each compound. This equation is called a skeleton equation and as such is considered incomplete. The equation must be balanced.
Balancing Equations
A skeleton equation does not conform to the Law of Conservation of Mass. Often times there are more atoms on one side of the equation
than the other. This is why equations must be balanced. In the above example there is one atom of hydrogen and one atom of chlorine on the reactant side,
and two of each on the product side. To correct this defficiency simply put a number "2" in front of the HCl molecule. The equation now
looks like:
Zn + 2HCl ------> H2 + ZnCl2
The number 2 is called a coefficient and it is the coefficients that you insert to "balance the equation".
Balancing equations is best done by simply doing it not learning a bunch of rules.
Worksheets and examples will be provided in class.
However you may go to the sites listed below and get help and many practice problems with answers!
How to Balance equations from Ohio State. Just click the OS logo. 
Practice Balancing Equations courtesy the Chembalancer
